Quantum Communication Experiment
Research overview
The Quantum Communication Experiment team studies technologies for generating, transmitting, and measuring quantum information with photons. Quantum networks require us to generate entanglement, transmit it reliably through optical fiber, and make photons from different locations interfere precisely before detecting them.
The Quantum Communication Experiment (QCE) team works on entangled-photon sources, entanglement swapping, phase stabilization, and wavelength multiplexing through experiments with optical systems and fiber. We test theoretically proposed communication schemes in real optical systems and study the technologies needed for future quantum networks.
Connecting entanglement through real optical fiber
Expanding quantum networks requires entanglement swapping, which generates entanglement between distant locations. In real optical fiber, temperature changes and vibration affect the phase of light, making stable interference over long periods a challenge. We use light at different wavelengths to stabilize the phase and conduct experiments to transmit and connect entanglement through long optical fibers.
Examples of our research: single-photon entanglement transmission through 10 km of optical fiber, and entanglement swapping using 3 km of deployed optical fiber
Developing entangled-photon sources for multiplexing
Efficient use of multiple wavelengths is important for connecting many users or channels at the same time in a quantum network. We develop polarization-entangled photon-pair sources compatible with wavelength-division multiplexing technologies used in telecommunications, and study how to use multiple frequency channels. By measuring the source spectrum and the quality of entanglement, we examine the conditions under which entanglement quality can be maintained during multiplexing.
Examples of our research: a polarization-entangled photon-pair source compatible with 50 GHz dense wavelength-division multiplexing, and entanglement swapping through frequency multiplexing
Using quantum and existing optical communication in the same network
Introducing quantum networks into practical communication infrastructure also requires technologies that carry quantum signals and conventional optical communication through the same fiber. Noise and crosstalk from the strong optical signals used in conventional communication can substantially affect quantum signals at the single-photon level. We experimentally study how these effects depend on wavelength allocation and fiber type, and how quantum communication can coexist with existing optical communication.
Examples of our research: coexistence of quantum and classical DWDM channels in hollow-core fiber, and crosstalk evaluation under wavelength multiplexing
How we conduct research
The Quantum Communication Experiment team designs optical systems to suit each research objective. We build experimental setups with lasers, optical fiber, optical components, and single-photon detectors, then generate and transmit quantum states and collect measurement data. As we analyze the results, we adjust the optical system and stabilization methods. During long measurements, controlling phase and polarization is also important. Some projects involve joint experiments with NICT and universities and research institutes in Japan and abroad.